It is AppleScriptable, and allows you to import addresses from tab-delimited files.

Here are some key features of "Envelope Window":
Address Lists
Envelope Window allows you to keep a list of addresses you frequently use, as well as multiple return addresses you may have. The lists are accessed through drawers that main window's right or left edge. You can select multiple destination addresses for batch printing. The 'tag' of an address is what shows in the list. At first, this is the first line of the address, but you can change it by control/right-clicking on the address and choosing 'Show Inspector'. You can also edit the address from here. You must click the Apply button to save any changes you make in the Inspector.

Recent Addresses
A list of your 5 most recently printed addresses appears in the popup menu located to the left of the destination address field. This is entirely independent of the address lists - whether or not an address appears in your regular list has no bearing on if it is listed here.

'Switch Addresses'
The Switch Addresses command is useful for printing a SASE (self addressed stamped envelope) to send along with your letter. It will switch the return and destination addresses, allowing you to print a new envelope addressed to yourself. Its keyboard shortcut is Command-`

Envelope Sizes
You can set an envelope size from the Envelope Size item under the Envelope menu. I've tried to include a number of common US and European sizes, but if there's one you need that isn't present, please email and let us know. Any settings made here override settings from your printer's Page Setup panel.

'Note' field
As a bonus for purchasing a license, after you enter your license code, the UNLICENSED text will disappear and you'll be able to enter a memo or 'attn:' line in that field, which will be printed in place of the 'Printed with...' line on your envelope.

Services
Envelope Window takes advantage of Mac OS X's Services functionality. Just select some text, then choose Services > Envelope Window > Print Envelope from the Application menu. Envelope Window will launch with your text filled in and bring up the standard Print panel for your approval. Note: Services will only be available if Envelope Window is located in your Applications folder.

On launch
If Envelope Window detects a small amount of text (like an address) on the clipboard when it starts up it'll insert it into the destination address field, which is quite probably what you were going to do next. If that isn't what you wanted, the text remains selected so you can just type over it to replace it.

What's New:
This is an interim release, mainly intended to correct the bug where the unlicensed notice would still print from copies that have been properly licensed. There are a few nonworking features (the drawer preferences) and not everything is finalized (the printing alignment controls will almost certainly be moving for the final release - it's those 3 buttons at the bottom of the Fonts & Printing preference tab).

The Envelope Sizes menu is gone, replaced with a panel to set and define your own sizes in the Preferences. It may be coming back, or there may be a menu in the Print sheet to set this without going into Preferences..

You can now position the addresses to your liking with the boxes below the envelope size area. It's important to remember that these are offsets from the standard positions. In other words, to move the destination address 18 points to the left of where it normally prints, enter -18 in the destination addresses Left box.

To get text to be inserted from the clipboard on startup as prior versions did, you may need to first turn it on in Preferences > Application Behavior. You can set the threshold where text will be considered too long to be an address in this tab, as well as the number of your most recent addresses that will be remembered.

The Font preferences will be used when you start up the application with no text on the clipboard to insert - anything you type will appear in your preferred font instead of the default Helvetica 12 point. Text that gets inserted from the clipboard on startup will retain its styling, unless you check the 'Use for clipboard text' box underneath the font settings area.